1. Field of the Invention
This invention relates to continuous direct casting of metal strip by a process employing a moving chill surface upon which molten metal is flowed for solidification beginning on the chill surface and progressing in a direction outwardly to the free top surface of the strip formed.
2. Prior Art
The advantages to be achieved in direct casting of molten metal into thin strip or sheet (hereinafter strip) on a continuous basis have long been recognized and numerous processes and devices have been proposed for the direct casting of metal strip. These prior art processes or devices have generally not been successfully employed on a commercial basis, however, particularly for the production of a high quality strip suitable for use in the as-cast condition for the production of commercial products, or for further processing as by rolling or shaping by other means.
In the known prior art continuous or direct strip casting processes employing a continuOusly driven liquid cooled chill surface which contacts a melt of the metal to be cast, the melt is solidified by extracting heat through the chill surface so that a thin skin of the metal is solidified immediately upon contact with the chill. This skin grows in thickness as the chill moves progressively through or past the melt until the strip is completely formed. The thin skin initially formed is firmly adhered to the chill and this intimate bonded contact results in a maximum heat transfer from the melt to the chill. This process is generally referred to as a melt drag process.
The extraction of heat from the progressively formed strip results in contraction of the solidifying strip at its bonded interface with the chill until the bond is broken, thereby producing a substantial reduction in the rate of heat extraction. The successful production of quality cast strip by such a process depends to a large degree upon the ability to extract heat at a uniform rate across the width of the strip to produce a substantially simultaneous release of the cast product from the chill across the full width of the strip.
Uneven or premature release of the forming strip and areas of uneven contact with the chill can result in localized surface defects. These surface defects may be manifested as discoloration, cracking, texture variation, surface porosity, thickness variations, and dimpling, some of which defects may appear on either or both surfaces of the strip. One cause of localized surface defects has been attributed to gases either evolving or entrained between the strip and chill, which gases expand and cause localized premature release.
The problem of localized uneven contact or premature release as a result of gases between the strip and chill surfaces has been largely solved by use of a grooved chill surface which provides continuous, unobstructed escape passages for any such gases. The size and configuration of the individual grooves is such that the melt does not penetrate to the root, leaving a gas escape passage at each groove. The spacing of the grooves is such that only a narrow band of the chill surface between adjacent grooves is in direct contact with the forming strip, thereby eliminating the possibility of entrapment of sufficient gas to cause premature release over an area sufficiently large to materially affect the cooling rate.
Defects which are not associated with localized premature strip release also frequently occur in the top surface of the strip, i.e., the surface away from the chill. These defects may include surface cracks as well as variations in gauge longitudinally of the strip and variations in transverse profile, making subsequent rolling or other processing of the cast strip difficult or impossible. Attempts have been made to control profile and gauge variations by contacting the unsolidified top surface of the strip with a second chill roll. Such cooled top rolls cool and solidify the top surface or skin of the strip and act as a flow controller to limit the amount of molten metal deposited upon the chill.
It is also known to contact the top surface with a pressure roll immediately after solidification of the strip in an attempt to control the strip gauge by a hot rolling action. Such pressure rolls have been internally cooled or otherwise configured to dissipate heat and provide a chilling effect on the strip.
Difficulty has been encountered in attempting to use a top roll of the type disclosed in the prior art in contact with the formed or forming strip moving on a primary chill surface. For example, when a cooled top roll is employed in contact with molten metal, there is a tendency for the top surface to solidify on contact with the chilled surface and to adhere to the surface in the same manner as the bottom strip surface adheres to the primary chill. Although this bond between the top strip surface and the cooled top roll may be only brief and broken more readily as a result of the relatively small radius of the top roll, cast sheet defects can nevertheless result.
Difficulty has also been encountered in attempting to form directly cast thin metal strip using a top roll engaging and hot rolling the top surface after solidification. Although the strip is still extremely hot and soft when contacted with the top shaping roll, metal movement in such a rolling operation can essentially only be in the rolling direction, i.e., longitudinally of the formed strip, and therefore corrections of the transverse profile of the strip cannot be accomplished. Further, any surface defects of the type described above will be formed before the strip reaches the top roll, making rolling difficult or impossible. Also, hot rolling can result in propagating surface cracks rather than eliminating them.
Attempts to shape the strip cast on a grooved chill surface by a hot rolling operation immediately following solidification on the chill also results in forcing of the bottom surface of the soft strip into any grooves or other surface depressions employed on the chill surface. This results in the grooved pattern of the chill being replicated on the bottom surface of the strip which can present problems in subsequent rolling or processing operations. Pressing the strip into the chill surface grooves also promotes non-uniform release and sticking or transverse tearing of the still soft strip.
It is, accordingly, a primary object of the present invention to provide a novel process of and apparatus for producing directly cast thin metal strip in a high speed commercial operation.
Another object is to provide such a process and apparatus to produce a directly cast thin metal strip having an improved transverse profile and more uniform longitudinal shape.
Another object of the invention is to provide such a process and apparatus having a smooth uniform top surface substantially free from transverse cracks.
Another object is to provide such a process and apparatus wherein thin, wide metal strip is directly cast on a grooved chill surface in a melt drag casting operation with an uncooled or heated top roll engaging the unsolidified top surface of the strip being formed to provide a strip of more uniform gauge and profile and having a top surface which is substantially smooth and free of transverse cracks.